Anisotropic thermal expansion in wurtzite-type crystals

The temperature dependence of the lattice parameters of six wurtzite-type c rystals, BeO, AlN, GaN, ZnO, CdSe, and CdS, has been measured by X-ray powd er diffractometry. The thermal expansion changes nonlinearly with temperatu re and is anisotropic; the expansion along the a-axis is larger than that a long the c-axis (more than 50% larger in ZnO, CdS and CdSe), resulting in a decrease of the c/a ratio with increasing temperature. The calculated Grun eisen parameters range between 0.27 and 1.45 and have a positive correlatio n with the bulk moduli. The anisotropic thermal expansion is associated wit h the anisotropies of both elasticity and anharmonicity. (C) 2000 Kluwer Ac ademic Publishers.